1. Field of the Invention
The present invention relates in general to the processing of materials into a usable form, and in particular to the creation of plastic slivers from plastic materials. The plastic slivers are especially suitable for use in composites of cellulosic and plastic materials.
2. Description of the Invention
The art of creating cellulosic/plastic composites is well-known; see, e.g., U.S. Pat. Nos. 5,082,605, 5,088,910, and 5,096,046, among others. Generally, two or more dissimilar materials, at least one of which is a thermoplastic, serve as the source material. The source materials are blended together and introduced into a carrier fluid, e.g. air. The carrier fluid passes through a screen and deposits the blended materials onto the screen in the form of a mat. After the mat has been formed it is pressed into flat panels or molded shapes using a heated press. During pressing the plastic melts and flows around the cellulosic fibers, thereby encapsulating them and creating a rigid structure. An additional chemical binder may be used if the plastic does not impart sufficient binding strength. Some typical applications for air-laid composites are interior automotive panels, which are later overlaid with vinyl or carpeting, and curved furniture components such as chair seats.
The depletion of our natural resources and increased problems with waste disposal are also well-known. The energy and raw material required to create plastics and the over-flowing of landfills make the recycling of plastics an attractive option to landfilling plastic waste. Using recycled plastics in cellulosic/plastic composites would therefore be appealing if an economically feasible method of preparing them for such use was available.
Several problems exist, however, in using recycled plastics in cellulosic/plastic composites. Different types of recycled plastics are often mixed together and contain contaminants such as food and labels. Prior methods of making plastic fibers for use in composites have generally involved melting the plastic as a step in the fiber-making process (i.e. as a step prior to extrusion). These methods require the plastic to be homogeneous and free of contaminants, or the different plastics (with different melting points) and any existing impurities will at the very least cause the process to go awry; at most, they may possibly ruin the apparatus used to make the fibers. Impurities can foul flow passages in machinery used for melting plastic, and mixtures of different melted plastics generally cannot be formed into fibers of uniform shape and quality due to irregularities in the melting temperatures and viscosity (and hence the flow properties) of the mixture. The greatest problems arise when blow molding grade plastics such as HDPE, with low fractional melt indices, are introduced into the mix; they cannot be formed into fibers by extrusion processes because their viscosities are too high to form a fiber when forced through a die. The cleaning and sorting steps in processing the plastic tend to be time-consuming and expensive, and the heating necessary to melt the plastic adds additional expense due to the energy-intensive heating process. Some fiber production methods are able to sidestep one or more of these requirements, but they invariably have their drawbacks. U.S. Pat. No. 4,107,827 describes a method for producing fibers from waste plastic stock by a mechanical process, rather than by a melting process, but it requires the plastic fiber stock to be pre-formed into a cylinder (and thus melted and possibly cleaned and sorted). U.S. Pat. No. 3,806,562 describes a method for producing stock for composites out of waste plastic without any cleaning or sorting steps, but it requires heating and extensive machining. U.S. Pat. Nos. 4,408,976 and 4,499,037 describe methods for producing plastic composite stock from plastic tubes, but they cannot be applied to stock with a non-tubular shape. The present invention resolves these problems by providing a method and apparatus for converting plastic waste into fibrous plastic composite stock which does not require melting, nor extensive cleaning, nor homogeneity of the waste plastic stock.